falseknees comics aren't necessarily structured around a big punchline. It's often more of an animal kingdom slice of life. Here the bird on the left clearly hatched this year since they're unaware of leaves changing color in autumn. This is isn't their last surprise, as they're about to find out what it means to be a migratory bird as well.

I think the bluer bird is younger, not knowing that summer ends, and being even more surprised that this involves immense journeys

I think the blue bird doesn't migrate, but idk. Just a guess.

Pretty sure it's just supposed to be one of those how is it already this late in the year type things

I think the joke is kind of like the talking muffin joke. One bird sees a leaf changing color and apparently has a pattern recognizing brain enough to identify that summer is ending and remembering from last year that that means they'll have to fly south.

The other bird is a bird and it's a miracle he recognized that the leaf changed color from day to day. He probably doesn't even remember what migration is, it's all instinct.